ROUGH SENSITIVITY ESTIMATES USING DIRECT SUN
1
ROUGH SENSITIVITY ESTIMATES USING DIRECT SUN BETWEEN 1.5 AND 2 AIRMASSES AIRMASS CHANGE = 0.5 CO 2 RATIO CHANGE=.024 SINCE 1 AIRMASS IS ~ 380 PPM WE GET 190/.024=7917. so A RATIO CHANGE OF .001 IS ABOUT 7.9 PPM FOR THE COLUMN AVERAGE 13 CO 2 Considerations on Validation of Space Based CO 2 Measurement Wm. S. Heaps 1 , E. L. Wilson 1 , E.M. Georgieva 1,2 1 NASA, Goddard Space flight Center, Greenbelt, MD 220771, phone: 301-286-5106, fax: 301-286-1750, e-mail: [email protected] 2 Goddard Earth Sciences and Technology Center, Baltimore, MD 21228 ABSTRACT The precision requirements for total column CO 2 measurement from space are more stringent than any ever attempted. Similarly the requirements on a validation system for a spaceborne CO 2 column measurement are very demanding. We at Goddard have been developing a Fabry-Perot based instrument for remote sensing of CO 2 column. This instrument holds high promise for offering a relatively inexpensive method for measuring CO 2 column from the ground at very high precision--necessary for validating space borne instruments. We shall present some results obtained using our instrument and discuss some of the factors necessary for making a measurement of total column CO with precision less than 1%. CONCLUSIONS •WE PRESENT RESULTS FROM GROUND TESTING OF A NEW INSTRUMENT CAPABLE OF VERY PRECISE MEASUREMENTS OF ATMOSPHERIC CARBON DIOXIDE, OXYGEN, AND WATER VAPOR •THE INSTRUMENT HAS HIGH SENSITIVITY TO SMALL CHANGES IN THESE SPECIES AND IS CAPABLE OF RAPID TEMPORAL RESPONSE. •TO ACHIEVE THE DESIRED DEGREE OF PRECISION FOR CARBON DIOXIDE COLUMN MEASUREMENTS GREAT CARE MUST BE EXCERCISED IN THE TRACKING OF THE SUN. •A LOW COST SOLUTION TO THE PRECISION TRACKING PROBLEM HAS ALSO BEEN DEVELOPED AND PRELIMINARY INDICATIONS SHOW ITS PROMISE AS A SOLUTION THE OVERALL TECHNIQUE IS APPLICABLE TO GROUND, AIR, AND SPACE BASED PLATFORMS. IT CAN BE EXTENDED TO ADDITIONAL SPECIES. •FUTURE WORK HIGHLY DESIRABLE AIMED AT VERIFYING TECHNIQUES FOR IMPROVING SENSITIVITY, ACHIEVING LONG TERM STABILITY, AND EXTENDING TECHNIQUE TO OTHER ATMOSPHERIC MEASUREMENTS Line strength for CO 2 isotopes 13 CO 2 and 12 CO 2 line strength from HITRAN spectra. •Spectroscopic methods will be used to make the global measurements •OCO (Orbiting Carbon Observatory) uses a large grating spectrometer •Plan is to use a network of FTIR spectrometers for validation. •FTIR spectrometer is commercially available. It can make a high resolution of the solar spectrum in a little less than 2 minutes. • CO 2 and O 2 column density can be derived from these spectra. •Our group at Goddard has been developing an instrument based upon the Fabry-Perot interferometer to make column measurements THE MISSING SINK • More CO 2 is being removed from the atmosphere than can be explained diagnostically. • Global measurements (ie. satellite) needed to locate sink • Precision requirements for a total column measurement are daunting (~.3%) • Ground based validation methods for satellite must meet or exceed this value The 3 species version of the Fabry-Perot instrument Recent Data, June 30, 2006 The FP to REF channel ratio is plotted as a function of local time for an O 2 instrument (yellow), a CO 2 instrument (blue), and a water vapor instrument (red).The principal variation is that arising from the change in airmass as the sun rises and sets throughout the day. The ratios from the previous figure are plotted against the calculated airmass. The AM and PM values for O 2 are virtually the same and are and cannot be distinguished. The CO 2 channel shows some slight variation throughout the day probably arising from consumption by plants. The water vapor channel shows considerable variation consistent with typical changes in humidity throughout a day. This figure shows the difference in the column average CO 2 measured to the bottom of the Sun minus the top of the Sun for various Solar Zenith Angles. THE SOLAR POINTING PROBLEM •SUN SIZE (~ .5 DEGREE) CAN BE BIG FACTOR IN AIRMASS(SEE FIGURE BELOW) •VIEWING WHOLE SUN RAPIDLY BLOWING THIN CLOUDS SIGNIFICANTLY CHANGE THE EFFECTIVE AIRMASS ON TIMESCALES OF A FEW SECONDS •TO ELIMINATE THIS EFFECT MUST VIEW ONLY A SMALL PART OF THE SUN REDUCING SIGNAL LEVELS AND PUTTING STRICTER REQUIREMENTS ON POINTING SYSTEM ACCURACY •FOR A SYSTEM LOCATED IN WASHINGTON DC THE SZA NEVER GETS <57 DEGREES IN DECEMBER-JANUARY •IMPLIES ELEVATION ACCURACY ~.1 DEGREES FOR 1 PPM PRECISION •SUN APPARENT MOTION PLUS CLOUD VARIABILITY COULD INTRODUCE SIGNIFICANT ERROR DURING 2 MINUTE SCAN OF FTIR SUN APPARENT SIZE IS ~.5 DEGREE AIRMASS TO TOP AND BOTTOM OF SUN CAN DIFFER SIGNIFICANTLY Intercomparison of H 2 O measurement by the Fabry-Perot and by a NOAA GPS based sensor located in Annapolis, Maryland for 5 days in late June early July. The FP only operates during the day. Agreement is consistent with variability expected for sites ~ 30 miles apart. SOLUTIONS TO THE PROBLEM •HIGH PRECISION POINTING IS EXPENSIVE AND DIFFICULT TO IMPLEMENT IN THE FIELD •WE HAVE DEVELOPED A “DITHERING TRACKER” THAT DELIBERATELY POINTS TO SEVERAL POSITIONS ABOVE AND BELOW THE SUN •SIGNAL STRENGTH CAN BE RELATED TO OVERLAP OF SUN WITH INSTRUMENT FOV ALLOWING CALCULATION OF POINTING POSITION Figure shows 2 minutes of data taken using dithering tracker. Red trace of reference channel power shows tracker central position is “high.” Blue trace of ratio shows that differences in airmass are ~80 ppm between top and bottom of sun. •DITHERING TRACKER DOESN’T WORK FOR FTIR BECAUSE SCANS TAKE TOO LONG. •FTIR SHOULD USE HIGH PRECSION TRACKER WITH SMALL INSTRUMENT FIELD OF VIEW (FOV) AND STOP TRACKING DURING SCAN
description
Considerations on Validation of Space Based CO 2 Measurement Wm. S. Heaps 1 , E. L. Wilson 1 , E.M. Georgieva 1,2 1 NASA, Goddard Space flight Center, Greenbelt, MD 220771, phone: 301-286-5106, fax: 301-286-1750, e-mail: [email protected] - PowerPoint PPT Presentation
Transcript of ROUGH SENSITIVITY ESTIMATES USING DIRECT SUN
ROUGH SENSITIVITY ESTIMATES USING DIRECT SUN
BETWEEN 1.5 AND 2 AIRMASSES AIRMASS CHANGE = 0.5 CO2 RATIO CHANGE=.024
SINCE 1 AIRMASS IS ~ 380 PPM WE GET 190/.024=7917. soA RATIO CHANGE OF .001 IS ABOUT 7.9 PPM FOR THE COLUMN AVERAGE
13CO2
Considerations on Validation of Space Based CO2 Measurement
Wm. S. Heaps 1, E. L. Wilson 1 , E.M. Georgieva 1,2
1 NASA, Goddard Space flight Center, Greenbelt, MD 220771,phone: 301-286-5106, fax: 301-286-1750, e-mail: [email protected]
2 Goddard Earth Sciences and Technology Center, Baltimore, MD 21228
ABSTRACT
The precision requirements for total column CO2 measurement from space are more stringent than any ever attempted. Similarly the requirements on a validation system for a spaceborne CO2 column measurement are very demanding. We at Goddard have been developing a Fabry-Perot based instrument for remote sensing of CO2 column. This instrument holds high promise for offering a relatively inexpensive method for measuring CO2 column from the ground at very high precision--necessary for validating space borne instruments. We shall present some results obtained using our instrument and discuss some of the factors necessary for making a measurement of total column CO2 with precision less than 1%.
CONCLUSIONS
•WE PRESENT RESULTS FROM GROUND TESTING OF A NEW INSTRUMENT CAPABLE OF VERY PRECISE MEASUREMENTS OF ATMOSPHERIC CARBON DIOXIDE, OXYGEN, AND WATER VAPOR
•THE INSTRUMENT HAS HIGH SENSITIVITY TO SMALL CHANGES IN THESE SPECIES AND IS CAPABLE OF RAPID TEMPORAL RESPONSE.
•TO ACHIEVE THE DESIRED DEGREE OF PRECISION FOR CARBON DIOXIDE COLUMN MEASUREMENTS GREAT CARE MUST BE EXCERCISED IN THE TRACKING OF THE SUN.
•A LOW COST SOLUTION TO THE PRECISION TRACKING PROBLEM HAS ALSO BEEN DEVELOPED AND PRELIMINARY INDICATIONS SHOW ITS PROMISE AS A SOLUTION
THE OVERALL TECHNIQUE IS APPLICABLE TO GROUND, AIR, AND SPACE BASED PLATFORMS. IT CAN BE EXTENDED TO ADDITIONAL SPECIES.
•FUTURE WORK HIGHLY DESIRABLE AIMED AT VERIFYING TECHNIQUES FOR IMPROVING SENSITIVITY, ACHIEVING LONG TERM STABILITY, AND EXTENDING TECHNIQUE TO OTHER ATMOSPHERIC MEASUREMENTS
Lin
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O2
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13CO2 and 12CO2 line strength from HITRAN spectra.
•Spectroscopic methods will be used to make the global measurements
•OCO (Orbiting Carbon Observatory) uses a large grating spectrometer
•Plan is to use a network of FTIR spectrometers for validation.
•FTIR spectrometer is commercially available. It can make a high resolution of the solar spectrum in a little less than 2 minutes.
•CO2 and O2 column density can be derived from these spectra.
•Our group at Goddard has been developing an instrument based upon the Fabry-Perot interferometer to make column measurements
THE MISSING SINK
•More CO2 is being removed from the atmosphere than can be explained diagnostically.
• Global measurements (ie. satellite) needed to locate sink
• Precision requirements for a total column measurement are daunting (~.3%)
• Ground based validation methods for satellite must meet or exceed this value
The 3 species version of the Fabry-Perot instrument
Recent Data, June 30, 2006The FP to REF channel ratio is plotted as a function of local time for an O2 instrument (yellow), a CO2 instrument (blue), and a water vapor instrument (red).The principal variation is that arising from the change in airmass as the sun rises and sets throughout the day.
The ratios from the previous figure are plotted against the calculated airmass. The AM and PM values for O2 are virtually the same and are and cannot be distinguished. The CO2 channel shows some slight variation throughout the day probably arising from consumption by plants. The water vapor channel shows considerable variation consistent with typical changes in humidity throughout a day.
This figure shows the difference in the column average CO2 measured to the bottom of the Sun minus the top of the Sun for various Solar Zenith Angles.
THE SOLAR POINTING PROBLEM•SUN SIZE (~ .5 DEGREE) CAN BE BIG FACTOR IN AIRMASS(SEE FIGURE BELOW)
•VIEWING WHOLE SUN RAPIDLY BLOWING THIN CLOUDS SIGNIFICANTLY CHANGE THE EFFECTIVE AIRMASS ON TIMESCALES OF A FEW SECONDS
•TO ELIMINATE THIS EFFECT MUST VIEW ONLY A SMALL PART OF THE SUN REDUCING SIGNAL LEVELS AND PUTTING STRICTER REQUIREMENTS ON POINTING SYSTEM ACCURACY
•FOR A SYSTEM LOCATED IN WASHINGTON DC THE SZA NEVER GETS <57 DEGREES IN DECEMBER-JANUARY
•IMPLIES ELEVATION ACCURACY ~.1 DEGREES FOR 1 PPM PRECISION
•SUN APPARENT MOTION PLUS CLOUD VARIABILITY COULD INTRODUCE SIGNIFICANT ERROR DURING 2 MINUTE SCAN OF FTIR
SUN APPARENT SIZE IS ~.5 DEGREE
AIRMASS TO TOP AND BOTTOM OF SUN CAN DIFFER SIGNIFICANTLY
Intercomparison of H2O measurement by the Fabry-Perot and by a NOAA GPS based sensor located in Annapolis, Maryland for 5 days in late June early July. The FP only operates during the day. Agreement is consistent with variability expected for sites ~ 30 miles apart.
SOLUTIONS TO THE PROBLEM•HIGH PRECISION POINTING IS EXPENSIVE AND DIFFICULT TO IMPLEMENT IN THE FIELD
•WE HAVE DEVELOPED A “DITHERING TRACKER” THAT DELIBERATELY POINTS TO SEVERAL POSITIONS ABOVE AND BELOW THE SUN
•SIGNAL STRENGTH CAN BE RELATED TO OVERLAP OF SUN WITH INSTRUMENT FOV ALLOWING CALCULATION OF POINTING POSITION
Figure shows 2 minutes of data taken using dithering tracker. Red trace of reference channel power shows tracker central position is “high.” Blue trace of ratio shows that differences in airmass are ~80 ppm between top and bottom of sun.
•DITHERING TRACKER DOESN’T WORK FOR FTIR BECAUSE SCANS TAKE TOO LONG.
•FTIR SHOULD USE HIGH PRECSION TRACKER WITH SMALL INSTRUMENT FIELD OF VIEW (FOV) AND STOP TRACKING DURING SCAN
